Resolution and
Print Size
by J. Ramón Palacios
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ppi and
dpi You are now the proud owner of both
a film scanner or a digital camera and an inkjet printer.
But may have wondered what is the best resolution -or pixel
density- of a digitized image to allow for the production
of a good print in a given size.
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| Building blocks of
a digital darkroom |
A printer's software translates
pixels into ink dots, that's why printer's resolutions are
expressed in dpi, dots per inch. How many
dots per inch is a printer capable of producing on paper is
a measure of its quality. In general, the more dpi, the smoother
is the tonal gradation in the print, the finer the definition
and the wider the color gamut.
On the other hand, a digital image from a scanner should be
stated in ppi or pixels per inch. For example,
specs for my Nikon CoolScan IV ED LS-40 state it is a 2900dpi
scanner. Looking into the effective reading resolution with
the MA-20 adapter for slides, clearance of 25.1 x 36.8mm,
the scanner yields images 2,870 x 4,203 pixels in size, or
12Mpixels. Roughly the equivalent to a 6Mpixels digital camera
image, as it seems from experience.
Doing calculations in the width
side: 2870pixels/25.1mm = 114.3426 pixels per mm or 114.3426
pixels per mm x 25.4mm per inch = 2904 pixels per inch, approximately
2900ppi.
Calculations on the length side
of the image scanned, reach the same result (4203pixelsx25.4mm
per inch/36.8mm=2901ppi, pixels per inch).
So it is ppi, not dpi.
This seems to be a common (translation?) mistake. My flatbed
Epson scanner also talks about dpi when it should be ppi.
Minolta, Hewlett Packard and other brand scanners they all
make the same misuse of the term. Strange.
A CoolScan 4000 ED will yield 3,946
x 5,782 pixels from a 35mm negative or unmounted slide, or
22.8Mpixels, roughly the equivalent to a 11.4Mpixel digital
camera image.
Digital images sizes from digital cameras are also stated
in pixels, like VGA, 640x480.
IMAGE RESOLUTION FOR PRINTING
Minimum resolution for magazine-quality
printing is 300ppi, so a VGA image of 640x480 will
only allow for a decent print of 3.2 x 1.6 inches (640pixels/300ppi=2.1
inches by 480pixels/300ppi=1.6 inches). 640x480=0.3Mpixels.
If you want to make a good 8x10
print under the above standards, it will be best to have a
300ppi image with a size of 2400 by 3000 (8x300 by 10x300),
a 7.2Mpixel scan. So now you know exactly the why of the quest
for higher Mpixel rating from scanners (and digital cameras),
even when interpolation may acceptably invent pixels where
there are none.
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